The invention relates to an accelerated method for the polymerization of lactams with simultaneous introduction of compounds which are able to affect the structure and properties of the resulting polyamide.
At the present time, most of the polymers of lactams are prepared by hydrolytic polymerization. Hydrolytic polymerization is usually carried out at an elevated pressure and requires high reaction temperatures and long polymerization times. Presently, processes which utilize anionic catalysts to promote polymerization have become popular. Processes which utilize anionic catalysts proceed more rapidly than hydrolytically catalyzed processes. (See U.S. Pat. No. 3,017,391.) However, the processes are very sensitive to the presence of small amounts of impurities which react with the catalyst system. The anionic polymer is somewhat less thermally stable due to the presence of residues of the strongly basic initiator. Anionic polymerization of lactams with high melting points, e.g., dodecanolactam, is technologically difficult because of the high polymerization rates achieved on mixing of the components.
A new initiation system has been sought which would overcome the shortcomings of the anionic and hydrolytic catalyzed polymerizations. The polymerization processes initiated with mineral and organic acids in an anhydrous medium have been published. (See G. M. van der Want, Ch. A. Kruisink: J. Polymer Sci. 35, 119/1959; T. G. Majury: J. Polymer Sci. 31, 383/1958.) However, carboxylic acids exhibit a smaller initiation effect than water. Strong mineral acids, such as HCl, initiate a very rapid polymerization but induce side reactions. The polymerization often stops before the polymerization equilibrium is achieved. (See S. Coubravsky, F. Geleji: Makromol. Chem. 105, 261/1967.) For this reason, the cationic catalysts have not yet been adopted in industrial use. The catalytic effect of Lewis acids, namely of BF.sub.3 and P.sub.2 O.sub.5, is mentioned in the literature (see M. Rothe: Angew. Chem. 80, 245/1968), but their catalytic effect requires the presence of water.
It is an object of the present invention to provide a cationic initiated polymerization process. It is a further object of the invention to provide a cationic initiated polymerization process which is carried out under substantially anhydrous conditions. Another object of the present invention is to provide a polymerization process which overcomes many of the deficiencies in the prior art processes.